Process and apparatus for filling microstructured fibers via convection based pressure driven technique

ABSTRACT

A delivery system for filling a microstructured or nanostructured optical fiber (MOF/NOF) includes a MOF/NOF having first and second ends. The first end of the MOF/NOF is disposed in a first chamber configured to have a first pressure, and the second end of the MOF/NOF is disposed in a second chamber configured to have a second pressure that is less than the first pressure. A material source introduces a material into the first chamber. A light source is oriented to emit light into one of the ends of the MOF/NOF. An optical detection system for monitoring filling of the MOF/NOF detects the light emitted by the light source from the other of the ends of the MOF/NOF.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application No. 61/171,881 filed Apr. 23, 2009, the entirety of which is herein incorporated by reference.

FIELD OF DISCLOSURE

The disclosed system and method also related to systems and methods for filling microstructured or nanostructured optical fibers. The disclosed system and method relate to detection systems using absorption spectroscopy.

BACKGROUND

Absorption spectroscopy exploits the characteristic absorption of radiation that corresponds to the wavelengths associated with the excitation of atoms in the sample. The characteristic absorption spectra are usually well defined making the detection method highly selective. The sensitivity of wavelength selective spectroscopy is dependent on the path length of the absorption process in the sample, concentration of the sample, and the line strength of the absorbing species. Typical detection limits for biological and environmental regulatory atomic absorption spectroscopy methods range from a few tenths of a part per million down to a few tenths of a part per billion. Selectivity qualities can be compromised by spectral interference, i.e., the overlapping of absorption lines from different samples.

Gas absorption characteristics are typically modeled as being governed by Beer's Law, which defines the relationship between the measured intensity of peaks of the absorption spectra to the physical conditions of the gas sample.

Conventional methods of microstructured optical fiber absorption spectroscopy rely on diffusion to fill an optical fiber with the gas or sample being tested. The concentration gradient between the concentration of the filling gas outside of the fiber and the gas inside the fiber drives the diffusion of filling gas molecules into the air molecules. The rate of diffusion is controlled by a classical binary diffusion model where the binary diffusion coefficient between the filling gas and the air can be used to estimate the filling time along with the fiber length. However, these conventional filling methods result in response times (i.e., filling times) on the order of minutes for fibers having lengths greater than or equal to one meter in length. Consequently, these conventional methods are impractical for implementation in sensors for use in industrial process monitoring, intelligent buildings, or other application where fast response times are needed.

SUMMARY

A delivery system for filling a microstructured or nanostructured optical fiber (MOF/NOF) includes an MOF/NOF having first and second ends. The first end of the MOF/NOF is disposed in a first chamber configured to have a first pressure, and the second end of the MOF/NOF is disposed in a second chamber configured to have a second pressure that is less than the first pressure. A material source introduces a material into the first chamber. A light source is oriented to emit light into one of the ends of the MOF/NOF. An optical detection system for monitoring filling of the MOF/NOF detects the light emitted by the light source from the other of the ends of the MOF/NOF.

In embodiments, the delivery system for MOF/NOF is configured as a sensor for analysis of the material introduced into the MOF/NOF.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram of one example of an improved filling system and/or sensing system using a microstructured or nanostructured optical fiber.

FIG. 1B is a block diagram of another example of a filling system and/or sensing system using a microstructured or nanostructured optical fiber.

FIG. 2 is a cross-sectional view of an optical fiber in accordance with FIGS. 1A and 1B.

FIGS. 3A-3D are spectrum graphs illustrating the absorption behavior of a gas in a photonic bandgap (PBG) fiber at various periods of time for a PBG fiber sensor in accordance with FIG. 1B.

FIGS. 4A-4B are graphs illustrating the fill time of a first PBG fiber in accordance with the PBG fiber sensor illustrated in FIG. 1B.

FIGS. 5A-5B are graphs illustrating the fill time of a second PBG fiber in accordance with the PBG fiber sensor illustrated in FIG. 1B.

DETAILED DESCRIPTION

An improved method and system for filling microstructured optical fibers (MOF) or nanostructured optical fibers (NOF), such as photonic bandgap (PBG) fibers, is provided. An improved fiber sensor, such as a photonic bandgap (PBG) fiber sensor, based on the improved filling technique is also described. The sensor has faster filling times compared to conventional PBG fiber sensors. In embodiments, the sensor includes a MOF or NOF fiber (e.g., a PBG fiber) having a first end disposed in a first chamber having a first pressure and a second end disposed in a second chamber having a second pressure. A gas (or other material for filling the fiber) is introduced into the first chamber, and a pressure differential resulting from the second pressure being less than the first pressure advantageously reduces the filling time of the PCB fiber with the gas compared to conventional diffusion methods. The pressure differential is sufficient to support forced convection, or put another way, pressure driven laminar convection. This forced convection can be modeled as a laminar flow. Light is emitted into an end of the optical fiber, and an optical detector receives light emitted from the light source at the other end of the fiber. The optical detector can be used to monitor the filling operation or a spectral analyzer can be provided for material analysis (e.g., material identification and concentration determination).

FIG. 1A is a schematic layout of one example of an improved MOF/NOF filling system 100A, which may be used for performing optical absorption spectroscopy as described below. As shown in FIG. 1A, filling system 100A includes a light source 102 coupled to a first gas housing 104, which defines a first chamber 106. A gas source 108 is hermetically coupled to the first chamber 106 through a pipe or other conduit 110 having a valve 112 disposed along its length. A MOF/NOF fiber 114, which is a PBG fiber in one preferred embodiment, has a first end 114 a disposed adjacent to light source 102 in the first chamber 106 and a second end 114 b disposed in a second chamber 116 defined by second gas housing 118. Second end 114 b of fiber 114 is disposed adjacent to an optical detection system 120, such as a photodetector or an optical spectrum analyzer (OSA) 120, such as a spectrometer, diffraction grating based optical spectrum analyzers, Fabry-Perot interferometer based optical spectrum analyzers, or Michelson interferometer based optical spectrum analyzer. A second pipe or conduit 122 is also hermetically coupled to gas housing 118 and includes a valve 124 disposed along its length. Secondary vents/gas ports for precision flow control can also be coupled to the gas housings via secondary valves 112, 124. Light source 102 may be a broadband light source or a narrowband light source such as, for example, a laser, a tunable laser diode or a light emitting diode (LED), respectively. Light source 102 is configured to transmit light into the first end 114 a of fiber 114, specifically into the core of the fiber 114. The light source 103 provides light having a wavelength within the absorption spectrum of the material provided into the MOF/NOF. In some embodiments, light source 102 provides near infrared light having a wavelength of approximately 1550 nm, although one skilled in the art will understand that light having other wavelengths may be provided by light source 102. In embodiments, the light source 102 arranged with respect to a window in the gas housing and aligned with the core of the fiber 114 a. In other embodiments, the light source can be coupled to a fiber in the gas housing by fiber splicing.

In an alternative embodiment, the light source need not be external to the gas housing 104. Moreover, the light source could be internal to the fiber 114. For example, the core of the fiber 114 could be provided with quantum dots which when energized emit light for transmission in the core of the fiber, similar to the operation of a florescent light.

Gas housings 104 and 118 may be any substantially air-tight housings defining internal chambers 106 and 116. In some embodiments, chamber 106 and 116 are disposed in a single housing and are separated by an airtight membrane or wall. A pump may be coupled to gas housing 104 and/or a vacuum may be coupled to gas housing 118 to create a pressure differential between first chamber 106 and second chamber 116. One skilled in the art will understand that other devices may be coupled to gas housings 104 and 118 to create a pressure differential between gas housings 104 and 118. In some embodiments, gas source 108 may be a pressurized gas source to provide the desired pressure differential between gas housings 104 and 118, which may limit the need for additional devices in the system for creating the pressure differential. Of course, any combination of a pressurized source, a pump and a vacuum could be used. While the gas source is illustrated as being in the first housing 104, this need not be the case. It should be understood that the gas can be introduced in the second gas housing 118 and flow away from the optical detection system toward the light source 102.

In embodiments, fiber 114 is a MOF fiber having a hollow core or microstructured cladding or lattice, for example a photonic crystal fiber (PCF) or PBG fiber. The length of the fiber 114 may vary between lengths of a fraction of a meter (e.g., less than 0.5 meters) to lengths in excess of hundreds of meters. The air cavities of these fibers provide a strong interaction of the optical field and the delivered material with low losses due to bending and other attenuation mechanisms.

Gas source 108 may provide a variety of gases to gas housing 104 for undergoing spectral analysis. The gas provided by gas source 108 may be sealed within gas housings 104 and 118 by closing valves 112 and 124, which are configured to be opened and closed to provide a hermetic seal as well as gas exhaustion. As described above, pressurized gas sources may be used to reduce the number of components in the system.

In embodiments, the optical detection system/analyzer 120 is configured to monitor the filling of the optical fiber by detection of the light emitted from the fiber. When the system is configured as a material sensor, the spectrometer 120 is configured to perform absorption spectroscopy on light transmitted from light source 102 that exits second end 114 b of PBG fiber 114. In one embodiment, spectrometer 120 is an optical spectrum analyzer (OSA) capable of detecting multiple-component gases based on the light transmitted by light source 102. The detection sensitivity of spectrometer 120 can be adjusted by varying the type of light source 102. For example, narrow bandwidth light sources with high intensities provide high signal-to-noise ratios for enhanced detection limits compared to broadband light sources. Broadband light sources are useful for systems that do not require narrow spectral linewidths or have high absorption coefficients.

The pressure difference between the chambers of the gas housings 104 and 118 increases the speed at which gas provided by gas source 108 fills PBG fiber 114, which results in shorter response times compared to conventional diffusion-based systems. Fill times of less than one minute have been achieved for PBG fibers 114 having lengths of less than one meter and pressure differentials of less than 15 psi through experimentation.

FIG. 1B is a block diagram of an experimental setup of a PBG gas sensor 100B. Like elements in FIG. 1B have the same reference numeral as those in FIG. 1A, and descriptions of like components are not repeated. As shown in FIG. 1B, sensor 100B includes a light source 102 coupled to a single mode fiber 126. Single mode fiber 126 has a hollow core surrounded by cladding having a refractive index that is less than a refractive index of the core. The core of single mode fiber 126 is sized and configured to internally guide and confine light provided by light source 102 in a single mode. Put another way, single mode fiber 126 has a core diameter that prevents higher order modes of the wavelengths provided by light source 102.

A first end 126 a of single mode fiber 126 is butt coupled to the first end 114 a of PBG fiber 114 in gas housing 104. The butt or direct coupling of single mode fiber 126 and PBG fiber 114 enables light waves transmitting along single mode fiber 126 to be coupled into PBG fiber 114 with minimal signal loss while enabling gas from gas source 108 to be received within PBG fiber 114. Gas source 108 is coupled to gas housing 114 through pipe 110 having valve 112 disposed along its length. A pressure transducer 128 is coupled between gas source 108 and valve 112, although one skilled in the art can appreciate that pressure transducer 128 can be located between valve 112 and gas housing 104 and/or provided to monitor the pressure of gas housing 118. Gas housings 104 and 118 define holes through which PBG fiber 114 extends such that the second end 114 b is disposed within the second chamber 116. Second end 114 b of PBG fiber 114 is butt coupled with first end 130 a of multi-mode fiber 130, which is coupled to optical spectrum analyzer 120.

Light source 102 was configured to output light having a wavelength centered at a 1550 nm. The alignment conditions of the light source 102 and fibers 126, 114, and 130 were monitored using a camera system to image the hollow core region at the fiber output to verify that the light was launched into the hollow core PBG fiber 114 at the fiber input from single mode fiber 126.

Nitrogen was used to purge PBG fiber 114 of residual gases and to obtain a spectroscopic baseline reading at OSA 120 for comparative purposes. The gas sample was admitted into the first chamber 106 at a designated pressure that was monitored by pressure transducer 128. Pressures were measured above ambient pressure at room temperature. The sample gas flowed through PBG fiber 114, which had its second end 114 b butt coupled to multi-mode fiber 130. The butt coupling of PBG fiber 114 and multi-mode fiber 130 enabled gas to exit PBG fiber 114 into chamber 116, which was vented to the atmosphere through valve 124. The transmitted output was measured by OSA 120 with a 0.2 nm resolution.

The spectral information was recorded at regular time intervals to monitor the filling dynamics of various lengths of PBG fiber 114. The sweep time, which was approximately one second, was constant for all measurements and was small compared to measurement intervals. Experiments were performed with C₂H₂ and CO₂ gas samples provided by gas source 108. The concentrations of the gas for each gas source 108 used in the experiment were certified by the manufacturer. Fiber lengths of 0.3 m, 0.7 m, 1 m, 1.9 m, and 27 m were tested.

In the tested gas regimes, the filling rates were assumed to be governed by a classical laminar flow. However, the regimes were following unsteady fluid dynamics theory, in which the filling gas displaced the resident gas in a “piston-like” fashion. For example, FIG. 2 illustrates a cross-sectional view of PBG fiber 114 having cladding 132 and a hollow core 134. Since the flow of gas through PBG fiber 114 was assumed to be fully developed, one-dimensional, and laminar, first order estimates were made of the filling time relative to the pressure differential between gas A and gas B within PBG fiber 114, which was a commercially available PBG fiber having a transmission window between 1480 nm and 1640 nm. It was also assumed that there was no mixing of gases as gas A filled up PBG fiber 114 and displaced gas B.

The second end 114 b of PBG fiber 114 was exposed to chamber 116 within gas housing 118, which was opened to atmospheric pressure such that the outlet pressure, P₂, was fixed. Due to the fact that PBG fiber 114 was connected to a fixed-pressure gas cylinder, i.e., gas source 108, the inlet pressure, P₁, was also fixed throughout the experiment. The properties of the gases (acetylene (C₂H₂), nitrogen (N), carbon dioxide (CO₂), and air (O₂)), such as viscosity, density, etc, were assumed to be constant. The density gradient of the gas is not significant for the range of pressures or volume of gas used to fill the length, L, of PBG fiber 114. The gradient may, however, affect the spectroscopic linewidths, which were not analyzed with the current system due to the broadening effects of multiple phenomena. The diameter, D, of the hollow core 134 of PBG fiber 114 was 12.5 μm.

The case in which the viscosities of the filling gas and the initial gas are similar to one another, the filling time linearly depends on viscosity. Consequently, the higher the viscosity of the gases, then the filling time will be higher compared to gases with lower viscosities. Additionally, the characteristic time for filling the fiber has an inverse relationship to the pressure difference, ΔP, and to the square of the diameter, i.e., D², of the PBG fiber 114 while having a direct relationship to the square of the length, L², of the PBG fiber 114. Thus, the diameter of the fiber core 134 and the length of the PBG fiber 114 have the most significant affect on the filling time.

The time period required for a gas to fill the length of a fiber can be determined from the velocity relationship for the speed at which L₁ (FIG. 2) is increasing.

As L₁ increases, the boundary between the two gases A and B within fiber 114 is displaced such that the speed at which L₁ is increasing approaches the rate of the delivered gas filling the PBG fiber 114

Spectroscopic data were collected for the different lengths of PBG fiber 114. The normalized absorption peaks of the characteristic spectra were monitored to observe the filling times, pressure broadening of peaks, and bend sensitivity of the gas-filled fiber 114, as expected.

Simultaneous detection of individual gases in a mixture using a fiber having a length of 27 m was spectroscopically confirmed. The measured spectra are consistent with existing reference spectra for sample gases. The gas mixture included 5% C₂H₂, 50% CO₂, and a balance of nitrogen. The absorption line strengths of CO₂ are significantly less (approximately three orders of magnitude less) than C₂H₂ in the spectral region of interest.

FIGS. 3A-3D are graphs illustrating the transmission spectrum of an unfilled PBG fiber 114 at various time intervals with a pressure of 17 psi and a noise level of approximately 90 dB. Specifically, FIG. 3A illustrates the transmission spectrum of the unfilled PBG fiber 114 at time t=0, when only air or nitrogen is present in the fiber; FIG. 3B illustrates the transmission spectrum of the PBG fiber 114 at time t=20 minutes; FIG. 3C illustrates the transmission spectrum of the PBG fiber 114 at time t=4.5 hours; and FIG. 3D illustrates the transmission spectrum of the PBG fiber 114 at time t=9 hours. As shown in FIGS. 3A-3D, the peaks 202 of C₂H₂ are initially pronounced (FIG. 3B) but then substantially disappear by the time the CO₂ peaks 204 are developed as shown in FIG. 3D.

Not only can the two gases be identified spectroscopically, but the concentration of each gas can be estimated by comparing absorbance measurements to a calibrated reference. Data were fitted to an exponential curve to approximate the time constants for the filling rate. Complete filling was assumed when 95 percent of the reference light from light source 102 was absorbed (i.e., T=3τ). Fiber lengths of approximately 2 m exhibited filling times of approximately 84 seconds for pressures of 13.5 psi as shown in FIG. 4A and filling times of approximately 120 seconds for pressures of 8 psi as shown in FIG. 4B.

The data also demonstrate that filling times increased with the length of PBG fiber 114, which identifies that the fibers were being filled with the sample gases. Each of the trials demonstrated that the filling rates are non-linearly dependent on fiber length.

The impact of pressure on the filling time is also demonstrated by the data. For example, the filing time constant can be reduced to τ=0.93 second at a pressure of 15.08 psi from τ=1.75 seconds at 10.28 psi for a fixed fiber length of 0.3 m as shown in FIGS. 5A and 5B.

The pressure-driven gas-filing technique described herein advantageously provides faster filling times compared to conventional diffusion-based techniques. A reduction in the time scales for filling have observed to be three orders of magnitude smaller than the conventional diffusion-based filling methods. Accordingly, reduced filling times may be achieved that enable longer fiber lengths to be used in systems such as sensor systems with increased sensitivity. Additionally, the reduced filling times enable PCB sensors to be used in a wide variety of applications including sensors to detect gas in engines, detection of fires, analyzing breath of a person or animal, detection of substances in airport or other security systems, and detection of a composition of a fluid sample for biomedical processing.

It should be understood that while the filling system has been illustrated in connection with gas filling of a MOF/NOF, such as a PBG fiber, the pressure driven filling system described herein can be used for filling MOF/NOF fibers with gasses, liquids, solids (e.g., gels) or combinations thereof. Moreover, as described above, the pressure differential between the ends of the fiber should be sufficient to induce convective flow, it should be understood that a combination of convective and diffusive flow may also be utilized. For example, the flow may be characterized at the ends of the fibers or at other localized regions as diffusive.

Although the invention has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended claims should be construed broadly, to include other variants and embodiments of the invention, which may be made by those skilled in the art without departing from the scope and range of equivalents of the invention. 

1. A delivery system for filling a microstructured or nanostructured optical fiber (MOF/NOF), comprising: a MOF/NOF having first and second ends, the first end of the MOF/NOF disposed in a first chamber configured to have a first pressure, the second end of the MOF/NOF disposed in a second chamber configured to have a second pressure that is less than the first pressure; a material source configured to introduce a material into the first chamber; a light source oriented to emit light into one of the ends of the MOF/NOF; and an optical detection system for monitoring filling of the MOF/NOF configured to detect the light emitted by the light source from the other of the ends of the MOF/NOF.
 2. The delivery system of claim 1, wherein the material source is a gas source and the first chamber is defined by a first gas housing and the second chamber is defined by a second gas housing.
 3. The delivery system of claim 1, further comprising a single mode fiber coupling the light source to the one end of the MOF/NOF.
 4. The delivery system of claim 3, further comprising a multi-mode fiber coupling the other end of the MOF/NOF to the optical detection system.
 5. The delivery system of claim 1, wherein the delivery system is configured as a material sensor, wherein the optical detection system includes an optical spectrum analyzer.
 6. The delivery system of claim 5, wherein the MOF/NOF is a photonic bandgap (PBG) fiber and the material source is a gas source.
 7. The delivery system of claim 1, wherein the MOF/NOF is a PBG fiber.
 8. The delivery system of clam 1, wherein the second pressure is atmospheric pressure.
 9. The delivery system of claim 1, further comprising a pressure transducer coupled to one of the first or second chambers.
 10. The delivery system of claim 1, wherein a difference between the first pressure and the second pressure is sufficient to establish convective flow of the material through the MOF/NOF.
 11. A filling method, comprising: providing a microstructured or nanostructured optical fiber (MOF/NOF) having first and second ends; establishing a pressure difference between the first and second ends of the MOF/NOF, the first end being at a higher pressure than the second end; introducing a first material to the first end of the MOF/NOF, the pressure difference being sufficient to establish convective flow of the first material toward the second end of the MOF/NOF; directing light from a light source into the MOF/NOF; and monitoring filling of the MOF/NOF using detection of the light directed from the light source.
 12. The method of claim 11, further comprising performing optical spectrum analysis on the detected light for material analysis.
 13. The method of claim 11, wherein the monitoring step includes monitoring absorption of the light transmitted by light source.
 14. The method of claim 11, wherein the MOF/NOF fiber is a photonic bandgap (PBG) fiber.
 15. The method of claim 11, wherein the light includes a wavelength within the absorption spectrum of the first material.
 16. A gas sensor system, comprising; a photonic bandgap (PBG) fiber having a first end disposed within a first chamber and a second end disposed within a second chamber; a gas source configured to introduce a first gas into one of the chambers; a light source configured to transmit light into the first end of the PBG fiber, the light having a wavelength within the absorption spectrum of the first gas; and an optical spectrum analyzer configured to receive the light transmitted by the light source from the second end of the PBG fiber, wherein the chambers are configured to provide a pressure differential between the first and second ends of the PBG fiber, the pressure differential being sufficient to establish a convective flow of the first gas within the PBG fiber for filling the PBG fiber.
 17. The system of claim 16, wherein the light source is a broadband light source.
 18. The system of claim 16, wherein a pump is coupled to one of the chambers for establishing, at least in part, the pressure differential.
 19. The system of claim 16, wherein a vacuum is coupled to one of the chambers for establishing, at least in part, the pressure differential.
 20. The system of claim 16, further comprising a single mode fiber coupling the light source to the first end of the PBG fiber and a multi-mode fiber coupling the second end of the fiber to the optical spectrum analyzer. 